Lead acid batteries are widely known as secondary batteries used in most vehicles. A typical lead acid battery may include several electrodes substantially submerged in an electrolyte (e.g., aqueous sulfuric acid). The electrodes include anodes, which may be made of an active material such as lead or a lead alloy, and cathodes, which may be made of an active material such as lead dioxide or another lead alloy. The electrodes chemically interact with the electrolyte to convert chemical energy into electrical energy and in some cases convert electrical energy into chemical energy. The electrodes typically include collection lugs.
Often, the electrodes are manufactured as pasted grids. Such electrodes may include a lead or lead alloy grid and a paste that includes red lead, dilute sulfuric acid and/or other additives, such as, for example, expanders. Paste may be provided on the grids and/or pressed into apertures defined by the grids and may then be dried or allowed to dry. Pasting paper may be provided on the electrodes during or after the pasting process.
Traditional batteries may also include separators provided between the electrodes. The separators may be made from, for example, wood, rubber, glass fiber, cellulose, sintered PVC/polyethylene, and/or any other known or later-developed insulating or non-electrically-conductive material.
A common occurrence in lead acid batteries is acid stratification. Acid stratification generally refers to the non-uniform concentration of electrolyte fluid within a lead acid battery. The electrolyte in a stratified battery concentrates toward the bottom, causing the upper half of the cell to be acid poor. Acid stratification may result from the battery being kept at a low charge without being fully charged during several charge/discharge cycles. For example, a vehicle that is only driven short distances often does not fully charge its starting, lighting and ignition (SLI) battery between successive starts of the vehicle. As a result, the battery may be maintained at a partial charge for an extended period of time and acid stratification may result. Acid stratification may reduce the performance of the battery and may eventually lead to a premature failure of the battery.
Acid stratification may also lead to sulfation in particular regions of the electrodes, such as for example on the lower portions of the electrodes. Sulfation generally refers to the formation of lead sulfate on one or more electrodes of a battery. Sulfation may result in crystallized lead sulfate formations that are difficult to break up or return to active material in the electrode and/or the electrolyte and may result in a loss of active material available to the electrode and the battery as a whole. Further, acid stratification and/or sulfation in a battery may result in the battery measuring a higher than actual open circuit voltage. As a result, the battery may appear to be fully charged when it actually may be only partially charged and may have a lower than expected cold cranking amps (CCA) value.
It has been found that uneven current density between two or more electrodes of a lead acid battery may contribute to acid stratification and/or sulfation within the lead acid battery.